Social bookmarking is a way for people to store, organize, search, and manage “bookmarks” of web pages. Users save links to web pages that they like or want to share, using a social bookmarking site to store these links. ... Examples of social bookmarking sites include del.icio.us and digg.com

Social Bookmarking site used in Seo.That are include in off page.we used for ranking and more organic traffic on web sites.Through Bookmarking We get a link so thats why we can used it Users save links to web pages that they like or want to share, using a social bookmarking site to store these links.